1. Field of the Invention
The present invention relates generally to regenerators, and more particularly to a regenerator usable in regenerative refrigerators and to regenerative refrigerators using the regenerator.
2. Description of the Related Art
Regenerative refrigerators such as Gifford-McMahon (GM) refrigerators and pulse tube refrigerators are capable of producing cold temperatures from low temperatures of approximately 100 K (kelvin) to cryogenic temperatures of approximately 4 K, and may be used for cooling superconducting magnets and detectors and in cryopumps.
For example, in GM refrigerators, working gas such as helium gas compressed in a compressor is introduced into a regenerator to be pre-cooled by a regenerator material in the regenerator. Further, after producing cold temperatures corresponding to work of expansion in an expansion chamber, the working gas again passes through the regenerator to return to the compressor. At this point, the working gas passes through the regenerator while cooling the regenerator material in the regenerator for working gas to be introduced next. Cold temperatures are periodically produced based on this process as one cycle.
In such regenerative refrigerators, a magnetic material such as HoCu2 is used as the regenerator material of the regenerator as described above in the case of producing cryogenic temperatures lower than 30 K.
Further, lately, studies have been made of using helium gas as a regenerator material of regenerators. Such regenerators are also referred to as helium-cooling type regenerators. For example, Japanese Laid-Open Patent Application No. 11-37582 illustrates using multiple thermally conductive capsules filled with helium gas as a regenerator material for a regenerator.
FIG. 1 is a graph illustrating changes in the specific heat of helium gas and the specific heat of a HoCu2 magnetic material relative to temperature. FIG. 1 clearly illustrates that at cryogenic temperatures around approximately 10 K, the specific heat of helium gas of a pressure of approximately 1.5 MPa is higher than the specific heat of the HoCu2 magnetic material. Accordingly, in such a temperature range, using helium gas in place of the HoCu2 magnetic material makes it possible to perform heat exchange more efficiently.
Practically, however, it is not easy to manufacture the capsule as illustrated in Japanese Laid-Open Patent Application No. 11-37582. For example, a pressure of approximately 160 MPa at room temperature is necessary in order for the helium gas in the capsule to have a pressure of approximately 1.5 MPa. A capsule filled with such high-pressure helium cannot be easily manufactured. Further, the formation of such a capsule resistant to high pressure inevitably results in an increase in the thickness of the capsule, thus reducing its thermal conductivity.
Therefore, lately, there has been a report of a helium-cooling type regenerator configured by providing multiple containers with holes inside the regenerator and causing helium gas used as the working gas of an apparatus to flow through the containers through the holes. (See Japanese Patent No. 2650437.)